1. Field of Technology
The invention relates generally to optical waveguides and particularly to optical coupling of a silica-based waveguide and a surface-mount device.
2. Related Art
Optical waveguide devices formed on planar substrates have become important elements for various optical network applications such as multiplexers and demultiplexers in dense wavelength division multiplexing (DWDM) systems and components in passive optical networks (PON). In the field of optical communications, use of monolithic silica-based planar lightwave circuits (PLCs) with optical passive devices is well known. FIG. 1 is a cross-sectional view of an optical waveguide device 1 formed on a planar substrate 10. On the planar substrate 10, there are a lower cladding layer 12, a core layer 14, and an upper cladding layer 16. These layers may be made of pure or doped silicon dioxide (SiO2). Light travels through the core layer 14.
With new developments in optical communication technologies, there is an increasing need for integration of optical, optoelectronic (e.g., laser diodes and photodiodes), and electronic components using low-cost passive alignment techniques to develop high functionality optoelectronics modules such as TOSA and ROSA. It is desirable to combine the low-cost silica PLC platform with an active device such as a laser diode or a photodiode to make a high-functionality optoelectronic module. This integration is difficult, however, because laser diodes and photodiodes are frequently made with III-V semiconductor substrates while PLCs are usually made with silicon substrates. While it is possible to make the PLC devices with III-V materials, this would make the integrated system expensive because III-V materials are more expensive than silicon.
The challenge in integrating the silica-based PLC with III-V-based active device lies in the interface. This challenge sometimes stems from the active device being a surface-mount device. Photodiodes, for example, are often surface-mount devices. To provide an effective interface, a method has been proposed whereby a micromirror using a silica-based PLC is used for optical path conversion. In this proposal, the micromirror is made of a resin by utilizing wettability control and surface tension effect. A well is first etched in the PLC and then the surface of a different area of the well is treated to make the contact angle of the resin on the surface different. The resin is put in the well by surface tension effect to form a mirror in the well. The mirror angle is controlled by the contact angles.
A problem with the above approach is that aside from the mirror groove, two termination grooves and a long resin supply groove are used to form the mirror. This increases the size of the mirror area. Also, the resin supply groove extends to the edge of the chip, making a deep groove along the chip. This deep groove along the chip may pose a problem for making electrode contact with the surface-mounted active device. This long groove may also affect the layout of the PLC waveguide.
A second approach is to fabricate an integrated mirror in a silica-based PLC. In this approach, a superficial layer is created by treating the surface in an oxygen plasma. During this treatment, the waveguide surface made of silica is subjected to intense ion bombardment by oxygen ions with an average energy of about 300 eV. Following this treatment, a layer of amorphous silicon is deposited as a hard mask. Then, chemical etching is carried out in a buffered (15%) hydrofluoric acid solution. Since the superficial layer etching rate is higher than the isotropic etching rate, a slope is formed in/on the waveguide. After depositing the aluminum as reflecting layer, a mirror is formed.
A problem with the second approach is that the mirror is formed by different etching rates in the surface-treated layer and the normal layer in isotropic etching. Because the surface-treated layer is usually shallow due to the limitations with ion bombardment, the mirror is usually short and covers a small part of the waveguide core layer. The small mirror size means only part of the waveguide mode is reflected, limiting the reflecting efficiency.